1. Field of the Invention
The present invention relates to a method of preparing trimanganese tetroxide (Mn3O4) nanoparticles from an exfoliated manganese dioxide (MnO2) nanosheet using a solid-state decomposition method, and Mn3O4 nanoparticles preparing according to the method.
2. Discussion of Related Art
In general, Mn3O4 powder is prepared by reduction of MnO2 or MnOOH or oxidation of Mn(OH)2. In the reduction process, Mn3O4 can be obtained by reducing a manganese oxide using a method of heating a manganese oxide such as Mn2O3, MnO2 or MnOOH in a kiln using methane gas as a reducing agent. When this process is applied, the heat treatment is carried out at 250° C. to 550° C. so as to prevent generation of MnO by excessive reduction. A method of yielding Mn3O4 powder using this process has an advantage in that the process is very simple. However, the method has problems in that it is difficult to prepare Mn3O4 powder in high purity since MnO is generated as impurities due to the excessive reduction during the heat treatment, and the reactivity is poor when used as a source material for preparing ferrite since a surface area of powder prepared by sintering Mn3O4 at a high temperature increases with an increasing particle size of the powder.
Another method of preparing a high-purity Mn3O4 powder includes a method of oxidizing a manganese metal powder in a pressurized reactor while the powder is dispersed in water. In this case, the method generally has advantages in that the reaction time is short, the prepared powder has high purity, and its particles are small and uniform in diameter. However, the method has problems in that the instrument is expensive, and it is uneconomic due to excessive use of energy since it requires high temperature and voltage.
Also, there is a method of preparing Mn3O4 which includes adding an alkaline solution such as an ammonia solution to an aqueous manganese sulfate solution to generate manganese hydroxide and adding an oxidizing agent to the manganese hydroxide. This method has an advantage in that the prepared Mn3O4 has a small particle diameter, but has problems in that wastewater containing sulfuric acid ions exuded from manganese sulfate is formed during the preparation of the Mn3O4, and thus it requires an additional process of disposing of the wastewater, and the sulfuric acid ions remaining in the Mn3O4 degrade product qualities as a magnetic material, and thus it requires an additional washing process to remove the sulfuric acid ions.
In recent years, a method of preparing Mn3O4 powder, which includes dispersing a metal manganese powder in an aqueous solution together with an ammonium salt selected from NH4Cl, NH4Br, NH4I, NH4NO3 and (NH4)2SO4 and introducing oxygen or an oxygen-containing gas as an oxidizing agent into the aqueous solution while heating the resulting mixture, has been known in the art. This method has problems regarding the corrosion of a reaction container by negative ions (Cl—, Br—, I—, NO3—, (SO4)2—, etc.) included in the ammonium salt, the wastewater disposal for removing negative ions, and the washing process used to prevent the negative ions from remaining in the final product, Mn3O4. Also, the method has problems in that the metal manganese powder may be condensed into lumps during the reaction as the sudden reaction is carried out on a surface of the metal manganese powder, and the purity of the condensed lumps of the metal manganese powder may be lowered due to insufficient oxidation of the metal manganese powder.